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A Historical Perspective of Synthetic Ceramic and Traditional Feldspathic Porcelain

Noble and base metal alloys possess several characteristics that make them useful in dentistry: strength, longevity, and biocompatibility, as well as the ability to be cast into the shapes and forms of natural teeth.1 They often lack, however, the one characteristic patients desire most—beauty—as they do not readily blend with the surrounding natural dentition. Today’s patients are continually made aware of the availability of aesthetic dental therapies through many forms of media, including magazines, newspapers, and television shows. But where does porcelain come from, when was it discovered, what is its historical significance? And why has it become one of the most widely used materials in contemporary aesthetic dentistry? These are the topics of discussion this article will present. 

 

Ceramic and Porcelain Fundamentals

Ceramics are formed by the union of metallic and nonmetallic elements.2 Most ceramic materials are oxides, formed by the combination of oxygen with metals and semimetals such as aluminum, calcium, silicon, and magnesium. Ceramic and porcelain are both broad terms for materials that humans utilize on a daily basis. Concrete, glass, fine crystal, and gypsum are all ceramics. Porcelain is a type of ceramic used extensively in dentistry among other industries. The structure of ceramics may be crystalline or noncrystalline. Crystalline ceramics (ie, quartz) have a regular arrangement of atoms in a lattice pattern. Noncrystalline ceramics (eg, granite) are typically amorphous in structure. In general, crystalline ceramics are very light and brittle, and amorphous ceramics are stronger and denser. All ceramics possess high melting points (ie, ~110°C to 1700°C) and low thermal and electrical conductivity.3 Additionally, because ceramics are oxides, they are fundamentally inert. This nonreactive quality provides synthetic ceramics with excellent biocompatibility. Human bone, teeth, and enamel are essentially biologically produced ceramics—the equivalent of which humans have been attempting to replicate synthetically for thousands of years.2

The Quest for “White Gold”

Translucent porcelain was first manufactured by the Chinese during the T’ang Dynasty (618-906 CE).4 Porcelain was so highly regarded, that the Chinese would neither divulge the ingredients, nor the correct proportions of those ingredients, for making porcelain. It was not long before its particular qualities were recognized and sought after outside China. Porcelain possesses a host of unique characteristics that set it apart from precious metals at one end of the spectrum, and transparent glass at the other. Of particular note is the way in which light passes through porcelain, making it luminous and seemingly internally lit. It is very different from the dense impenetrability of stone and earthenware clays. It is also different from glass, which light passes through with glittering clarity. Potters, who tried to find the secret of manufacturing porcelain, attempted to add glass to the clay so as to gain its unique translucent character. Attempts to make translucent porcelain by incorporating glass were made as early as the 15th century in Venice, Italy.4 Other similar experiments in porcelain making appear to have been performed in France in 1673 at Rouen and then at St. Cloud. The characteristics of this porcelain, known as pate tendre (soft paste) in France and as frit porcelain in England, are different from the true hard-paste porcelain of the Chinese. The popularity of soft-paste porcelain was short-lived, mainly because it had too many practical disadvantages. It was difficult to handle, frequently slumped, and distorted (interestingly, this is the same terminology used to describe the working properties of dental porcelains today), and had a tendency to melt during firing. The finished pieces were easily scratched, and tended to crack when exposed to hot liquids.

The elusive formula for making porcelain was rediscovered by Böttger, a German alchemist who stumbled upon it while imprisoned by Poland’s King Augustus.5 Between 1708 and 1710, Böttger invented fine stoneware that could be burnished on a lapidary’s wheel, but was ordered by the king to make a hardpaste material instead of merely imitating and refining earthenware. Around 1710, he found the proper balance of materials to mix into what was to be the first true European porcelain, white with a smooth texture and translucent quality identical to the porcelain of China.5 In 1720, Böttger died of serious health complications, most likely pneumonia, as his lungs were ravaged by the inhalation of noxious fumes. Böttger’s legacy lives on, however, as his formula for making porcelain remains the basis for the manufacture of much of the porcelain used worldwide today. 

(Continued from page 1 )

Porcelain in Dentistry

For nearly 1,000 years after its discovery, porcelain was used primarily to produce fine dishware and utensils. It was also used to create objects of art and jewelry for those who could afford it. In 1723, Fauchard first used porcelain to enamel the metal bases of dentures. He is also credited with recognizing the potential of porcelain enamels and initiating experiments that would lead to further advances in the use of porcelain in dentistry. In 1774, Duchateau experimented with dentures fabricated with hard porcelain; 20 years later, de Chament improved Duchateau’s process and secured a patent for “mineral teeth,” which became the first denture teeth.6 In 1885, Logan introduced the Richmond Crown, in which porcelain was fused to a platinum post; a year later, Land made the first fused porcelain inlay and crown backed by platinum foil.6 The use of porcelain as a viable restorative option in dentistry gained little further momentum until 1949, when the Dentist’s Supply Company of New York invented the vacuum firing of dense and translucent porcelain teeth. In 1958, the first dental porcelain for veneering was introduced, which led to the widespread use of metal-ceramic restorations in the 1960s and beyond, followed by the invention of the porcelain jacket crown that was popularized in the 1960s by McLean.6 The 1970s saw the advent of early experiments in CAD/CAM crown fabrication, followed by an influx of ceramic-based restorative systems from the 1980s through to the present day. 

 

Natural Feldspathic and Fully Synthetic Porcelains

Dental ceramics are essentially oxide-based glass ceramic systems. They possess three essential features: ease of fabrication of complex shapes, sufficient mechanical and corrosion resistance, and aesthetic appeal. Tremendous advances have been made in the mechanical properties and fabrication methods of these materials over the last few decades. The earliest successful porcelain systems used conventional feldspathic porcelain derived from feldspar, a natural mineral. While porcelain-based materials are a major component of the entire market, there is a trend toward all-ceramic systems in lieu of metal-cored systems.2  

The basic composition of feldspathic porcelain is a mixture of feldspar, potash, quartz, and kaolinite.7 Feldspar comprises up to 65% of the mix, followed by quartz (~25%) and metal oxides (~10%). Sodium oxide and alkaline earth oxides are added as bivalent glass modifiers to improve translucent properties and maintain strength. Fluxing agents are also added to lower the melting temperatures, thereby making feldspar-based porcelains easier to handle in the laboratory. Potassium oxide (K2O) is a major component of feldspathic porcelains, as it is used to give the porcelain a coefficient of thermal expansion that most closely matches that of the metal alloys used in dental metal-ceramic techniques. Leucite is also used as a constituent of dental ceramics as it, too, helps modify the coefficient of thermal expansion to comply with that of the metal to which it is fused or baked. Additionally, an increased volume of fine leucite particles imparts increased fracture strength.8

A new generation of fully synthetic quartz glassceramics (FSQGCs) that overcomes some of the disadvantages of traditional feldspathic-based ceramics has recently been introduced (Figures 1 and 2). Natural quartz inherently contains structural impurities that cannot be removed by conventional firing, whereas the synthetic quartz manufactured possesses a purer and more homogenous consistency (Figure 3).9,11 The synthetic quartz crystals are also subjected to a burn-out phase, where they are subject to a temperature of 1600° C, just below their melting point (1670° C), which vaporizes and removes all impurities.9 This process results in a ceramic material with optical purity, as well as fluorescence, opalescence, and translucency nearly identical to natural dentition (Figures 4-5-6-7-8-9).10 A microfine leucite crystal dispersion throughout the quartz matrix creates an extremely smooth surface with added properties of superior polishability and low wear resistance, similar to that of natural enamel.10 A very low firing temperature of 870° C allows the use of various coping materials that range from nonprecious to high-noble alloys.

The FSQGCs incorporate leucite crystals that are prematured. This prevents uncontrolled crystallization while providing a stable coefficient of thermal expansion. Therefore, unlike most natural ceramics, FSQGCs can be cooled much more rapidly, as well as fired multiple times without the risk of fractures and cracks.11 Traditional feldspathic ceramics have a tendency to change color when they are subjected to multiple firing cycles. This is due to the burnout of shade-defining metal oxides added to the raw material at the time of manufacture. To preclude this unwanted effect with synthetic ceramics, proprietary colorant crystals are added. The colorants are presintered at a temperature that is higher than the firing temperature of the ceramic powder, so that they do not vaporize during firing, and thus the color of the ceramic remains unchanged.9 Additionally, a presintering phase preshrinks and stabilizes the crystals so that shrinkage is not a factor upon subsequent firings.11

 

Conclusion

Since the earliest days of dentistry, dental professionals have sought to use restorative materials that offer realistic aesthetics, functionality, biocompatibility, and longevity. Nearly a millennium had passed since the discovery of natural porcelain before dentists began experimenting with ways to incorporate ceramics into their restorative armamentarium. Recent developments in the manufacture of synthetic quartz-glass have led to a new generation of fully synthetic dental ceramics that overcome some of the shortcomings inherent to feldspathic and other silica-based natural ceramics. This new class of fully synthetic ceramics provides excellent optical characteristics, increased strength, easier handling, simplified fabrication processes, and improved functionality. 

 

* Clinical Associate Professor and Director of the Continuing Education Program in Advanced Aesthetic Dentistry, New York University College of Dentistry, New York, NY; private practice, New York, NY. 

References

  1. Ferracane JL. Materials in Dentistry: Principles and Applications. Philadelphia, PA: Lippincott Williams & Wilkins; 1995:9-13. 
  2. Ironside JG, Swain MV. A review and critical issues of dental ceramics. J Aust Ceram Soc 1998; 34(2):78-91. 
  3. Burton W. Porcelain, Its Art and Manufacture. B. T. Batsford, 1986;17. 
  4. Rogers M. Mary Rogers on Pottery and Porcelain. New York, NY: Watson-Guptill Publications, Inc; 1979:135-142. 
  5. Gleeson J. Arcanum: The Extraordinary True Story. Vol 4 [audiotape]. New York, NY: Time Warner Audio Books; 1999. 
  6. Kalanta K. A crash course in ceramics. Available at: http://www.bethesda.med.navy.mil/careers/postgraduate_den tal_school/comprehensive_dentistry/Pearls/Pearlsa2.htm. Accessed on March 19, 2005. 
  7. Van Noort R. Introduction to Dental Materials. London, UK:Mosby;1994:234. 
  8. Blatz MB. Long-term success of all-ceramic posterior restorations. Quint Int 2002;33:425-426. 
  9. Chu SJ, Ahmad I. Light dynamic properties of a synthetic, lowfusing, quartz glass-ceramic material. Pract Proced Aesthet Dent 2003;15(1):49-56. 
  10. Fiechter PA. Discovering the esthetic code. Dent Dialogue 2004;4(1):1-13. 
  11. Chu SJ. Use of a synthetic low-fusing quartz glass-ceramic material for the fabrication of metal-ceramic restorations. Pract Proced Aesthet Dent 2001;13(5):375-380.
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